Computed tomography, particularly x-ray computed tomography (CT), is a widely used volumetric imaging principle. In general terms, a radiation source and a radiation-sensitive image sensor are arranged on a line, with the subject of the examination positioned in between. The subject attenuates the radiation. The source-detector arrangement is typically moved into several positions, often on a circle or segment thereof, around the subject of the examination, and images are taken at every position. The spatial, volumetric distribution of the attenuation coefficient within the subject can then be reconstructed from all images, for example using the filtered back projection algorithm, generating a 3D digital model of the subject. Often, the image sensor is a 2D sensor, such as in cone beam computed tomography (CBCT). In medicine, x-ray CT scanners are valuable non-invasive diagnostic devices.
A major concern related to the use of CT scanners in medicine is radiation dose. Accordingly, a large body of research has focused on volumetric reconstruction algorithms that exploit the image data in an optimal way, allowing fewer images to be taken, or a lower dose per image, for a given quality of the reconstruction. While filtered back projection is a direct algorithm, many refined algorithms are iterative ones. Because the volumetric reconstruction problem is ill-posed, various regularization approaches have been suggested, e.g., total variation. Maximum-likelihood estimation has also been proposed, for example with a prior based on material assumptions. Several proposed reconstruction algorithms contain some of the above elements, or all of them.
Another way to lower the needed dose in a CBCT system is to make sure the patient does not move during image acquisition. This is because for a given needed accuracy, the signal-to-noise ratio will be greater when the patient does not move. Also, when the patient moves, motion artifacts such as for example streaks and aliasing may deteriorate the image quality. Therefore, in general, the image quality will be better when patient movement is kept to a minimum.
In prior art CBCT systems, various forms of head fixation devices have been employed to keep the patient fixated during the x-ray recording. These systems all have the goal of minimizing effects from motion blur and patient movement, thereby achieving a higher accuracy of the final images. However, all these systems have the disadvantage that it may be uncomfortable for the patient to be fixated for the duration of the scan, in particular for patients that may suffer from claustrophobia. It therefore remains a problem to achieve a high accuracy of CBCT images without having to fixate the patient.
Yet another way to lower the used dose in a CBCT system is to accurately define a region of interest (ROI). Many existing CT scanners allow the selection of a region of interest, but the selection is made schematically, and in order to ascertain that the relevant volume is covered, the region of interest suggested by the system is often larger than necessary for a particular patient. Also, since there is a risk that the patient moves during x-ray image acquisition, the chosen ROI is often larger than ideally necessary, in order to make certain that the whole ROI is covered in a single scan. It may be useful if the CT scanner allowed realization of the adjustment of the region of interest by for example four independent collimator shutters for the top, bottom, left, and right sides of the beam.